The Drowning Pool by Ross Macdonald

Maude Slocum is in trouble. But luckily trouble is Investigator Lew Archer's business.

A well-dressed, wealthy woman has arrived at Archer's L.A. office, having intercepted a poison pen letter accusing her of adultery. Reluctantly agreeing to help her find the culprit, he dives into the Slocums' moneyed, oil-rich California world. But when Maude's mother-in-law is found dead in the swimming pool, secrets come to the surface too. For the urbane, world-weary Archer, a case of blackmail soon becomes murder.